1. Field of the Invention
The present invention relates to a DC-DC converter employing a synchronized-rectification system and, more specifically, to control of switching devices in the synchronized-rectification system.
2. Prior Art
With reduction of voltages supplied to integrated circuits, a small-sized converter capable of supplying a low voltage at high efficiency is now required. To provide such a converter, the synchronized-rectification system is widely employed in which a rectifying circuit is comprised of metal-oxide-semiconductor field-effect transistors (MOSFETs) which are caused to operate in synchronization with a primary switch of the converter.
FIG. 1 is a circuit diagram showing a conventional synchronized-rectification type converter. A MOSFET 1 as a primary switch is connected to the primary of a transformer TR. A rectifying circuit comprising a rectifying diode 2 and a flywheel MOSFET 3 is connected to the secondary of the transformer TR, and a smoothing circuit comprising a choke coil 4 and a capacitor 5 is connected to the rectifying circuit.
Monitoring a DC output voltage of the converter, a control circuit 6 controls the resistance of a variable-resistance device 7 in accordance with the level of the output voltage. The variable resistance device 7 and a resistor 8 are connected in series to each other to form a voltage divider, and divide a reference voltage V.sub.ref to form a control voltage V.sub.1, which is input to a comparator 9. The comparator 9 compares the control voltage V.sub.1 with a triangular-wave voltage that is output from a triangular-wave generator 10. A high-level voltage is applied to a gate of the MOSFET 1 and an inverter 11 when the triangular-wave voltage is not smaller than the control voltage V.sub.1 and a low-level voltage is applied to them when the triangular-wave voltage is smaller than the control voltage V.sub.1. An output of the inverter 11 is applied to a gate of the MOSFET 3. Therefore, the flywheel MOSFET 3 operates in synchronization with the MOSFET 1 such that one is in ON state when the other is in OFF state.
When the output voltage of the converter increases, the control circuit 6 decreases the resistance of the variable-resistance device 7, to thereby increase the control voltage V.sub.1. As a result, the period during which the triangular-wave voltage is higher than the control voltage V.sub.1 is shortened. In other words, the pulse width of a switching pulse that is output from the comparator 9 is shortened. Therefore, the MOSFET 1 as the primary switch operates to reduce the power transferred from the primary to the secondary of the transformer TR, to thereby decrease the output voltage of the converter. Conversely, when the output voltage of the converter decreases, the control circuit 6 increases the resistance of the variable-resistance device 7, to thereby decrease the control voltage V.sub.1. As a result, the pulse width of the switching pulse that is output from the comparator 9 becomes longer, so that the output voltage of the converter is increased. In this manner, the output voltage level of the converter can be kept constant by controlling the ON-period of the MOSFET 1.
However, because of a delay in operation of the inverter 11 and a delay in operation resulting from the gate capacitance of the flywheel MOSFET 3, the operation timing of the flywheel MOSFET 3 inevitably delays from that of the MOSFET 1. As a result, in the conventional converter, there may occur a phenomenon that, in a high-frequency switching operation, the primary switch MOSFET 1 and the flywheel MOSFET 3 are rendered in ON state at the same time, resulting in reduced efficiency of the converter.
Even if a MOSFET is used in place of the rectifying diode 2 and is controlled by the switching pulse that is output from the comparator 9, the reduction in the efficiency still occurs. This is so because there occurs a period during which both of the rectifying MOSFET and the flywheel MOSFET 3 are rendered in ON state, and the secondary of the transformer TR is short-circuited during that period.